1. Field of the Invention
This invention relates to odd order bandpass filters with at least one triple mode waveguide cavity. In particular, this invention relates to odd order bandpass filters having a triple mode cavity where there is at least one resonant feedback coupling created in the triple mode cavity. Further, this invention relates to an odd order bandpass filter where there are a plurality of cascade dual and triple mode waveguide cavities.
2. Description of the Prior Art
It is known to have odd order filters that produce an elliptic function response as set out in U.S. Pat. No. 4,246,555, naming Albert E. Williams as inventor and entitled "Odd Order Elliptic Function Narrow Bandpass Microwave Filters". Unfortunately, the filter described in said patent allows for only one single mode of propagation per cavity, thereby making the structure relatively large when compared to the present invention. It is also known to have dual mode cylindrical and/or cuboid filter structures that can be used to produce an elliptic function response as described by Atia, Williams and Newcomb in an article entitled "Narrow-Band Multiple-Coupled Cavity Synthesis", published in the Institute of Electrical and Electronics Engineers, Transactions on Circuits and Systems, Vol. CAS-21, No. 5, dated September, 1974, pp. 649 to 655. However, dual mode filters are also relatively large when compared to filters of the present invention. Further, more favourable results can be achieved with filters of the present invention than with prior filters.
Presently, it is common to use six-pole dual mode quasi-elliptic filters in continuous output multiplexers for satellite communications. Weight and volume savings are very important in satellite communications. Also, it has been found that five-pole odd order quasi-elliptic filter design can be used to provide better electrical performance than a six-pole dual mode filter. Further, when a five-pole filter design uses a triple and dual mode cavity, one cavity can be eliminated when compared to the six-pole dual mode design. This can result in a weight reduction of approximately 25% and a volume reduction of approximately 30%.